Interference color generating devices are well known in the art. For example, U.S. Pat. No. 4,837,061 issued on Aug. 10, 1987 to Paul Smits et al. discloses a layered tamper evident structure which exhibits an irreversible color change when the layers are separated, thus providing evidence that the structure has been tampered. FIG. 1 is an illustration of such an interference color generating device.
The device 20 of FIG. 1 consists of a layer 10, preferably of a valve or refractory metal, a thin film 12 of a light transmitting material in direct and intimate contact with the layer 10, and an overlying strip 14 of flexible, tensionable translucent or transparent material, e.g. polyethylene. Valve metals, including tantalum (Ta), niobium (Nb), zirconium (Zr), hafnium (Hf), and titanium (Ti) and refractory metals including tungsten (W), vanadium (V), and molybdenum (Mo), are capable of generating intense colors when covered by a thin film of light transmitting materials. Layer 10 and thin film 12 are capable of generating a color by a light interference and absorption phenomenon that requires direct and intimate contact between layer 10 and thin film 12.
The preferred method of forming the thin film of light transmitting material 12 is anodization carried out in the presence of an adhesion-reducing agent, preferably a fluorine-containing compound. The anodization process forms a thin film 12 made of an oxide of the metal used to form metal layer 10. Ta and Nb are particularly preferred metals because of the wide range of colors accessible with this technique. The metal layer 10 and oxide layer 12 are in such intimate contact that the two layers conform closely with each other at the microscopic level at the interface or structurally merge together at the interface of the two layers 10, 12. Additionally, metal layer 10 and oxide layer 12 are in such direct contact that essentially no other material is between the two layers 10, 12 at the interface and excludes the presence of gas molecules from the air that tend to adhere to the layers once they are separated. The adhesion of the overlying strip 14 to the thin film 12 should be greater than the adhesion between the film 12 to the layer 10.
Referring to prior art device 20 in FIG. 1, white light incident on the structure, indicated by ray A, is partially reflected by the upper surface of the thin film 12 (ray B) and is partially transmitted to be reflected (ray C) by the upper surface of the layer 10. The interference colors generated when rays B and C combine will be weak if the relative intensities of rays B and C differ significantly, but will be intense if the intensities of rays B and C are similar. When highly reflective metals are used for the layer 10, most of the light is reflected at the upper surface of the metal layer and so ray C is much more intense than ray B. Light absorption (indicated by arrow X) takes place at the interface between thin film 12 and the layer 10. This absorption reduces the intensity of ray C and makes the intensities of rays B and C more comparable so that an intense color is generated. The light absorption depends on direct and intimate contact between layer 10 and film 12.
Separation of layer 10 and thin film 12 eliminates the light absorption indicated by ray X. As a result, the intense color originally generated by rays B and C is lost, leaving the gray color of the material 10 generated by ray C. The color generating device 20 is therefore also referred to as a color change device or a color changeable device. The intense color originally formed by rays B and C cannot be regenerated by repositioning film 12 on layer 10, even if the layers are pressed together, because the contact will no longer be direct (gas molecules intervene) and/or intimate (the surfaces will no longer conform closely at the microscopic level). As a result, the substantially irreversible color change acts as evidence that the layers have been separated and consequently that the structure has been disturbed or tampered.
U.S. Pat. No. 4,837,061 discloses separating layers 10, 12 in the device 20 shown in FIG. 1 by peeling the transparent or translucent material 14 and by puncturing by means of a needle or knife. This separation causes an irreversible color change.
U.S. Pat. No. 5,135,262, issued on Aug. 4, 1992, to Gary J. Smith et al. discloses an alternative means of separating layer 10, 12 by bending the laminate to small radii of curvature. FIG. 2 illustrates an interference color generating device as disclosed in U.S. Pat. No. 5,135,262. As the device 20 separates from article 22, its overall thickness and stiffness usually prevents it from forming a completely sharp angle, but instead it is bent around a short radius of curvature "r" at the apex of included angle ".alpha.." The concentration of adhesion-reducing agent used in forming the anodized film 12 on metal layer 10 is sufficient to permit color change activation when r and .alpha. are in the range inevitably encountered when peeling the entire device 20 from article 22. To allow separation of thin film 12 from metal layer 10 on bending, U.S. Pat. No. 5,135,262 teaches using concentrations of fluoride in the range of 40-350 ppm during anodization. When the fluoride concentration falls outside this range, the desired separation color change may not occur on bending. In the case of higher concentrations, thin film 12 may fall off prematurely leading to unwanted color change.
U.S. Pat. No. 5,062,928 issued on Nov. 5, 1991 to Gary J. Smith discloses a process for producing color-change devices incorporating latent indicia, which are initially invisible messages, pattern or designs. Upon separation of layer 10 and thin film 12 the latent message or pattern is made visible to indicate tampering.
The process disclosed in U.S. Pat. No. 5,062,928 for producing color changeable devices incorporating latent indicia includes the following steps: a) preparing a substrate having a very thin sputtered layer of metal; b) applying masking material to certain areas of the metal layer, the masking material comprised of printing inks or conventional uncured resist materials; c) submitting the substrate to a single step anodization process with an electrolyte containing adhesion-reducing agent to produce a color-generating laminate incorporating an anodic film having detachable and non-detachable areas; and d) removing the masking material by washing.
FIG. 3 shows color change device 20 made by the process disclosed in U.S. Pat. No. 5,062,928. The areas of the device which were covered with the masking material are visually indistinguishable from the uncovered areas when layer 10 and thin film 12 are in intimate contact. Despite this, the difference between the masked areas 12b and uncovered areas 12a is that the adhesive-reducing agent in the electrolyte has weakened the adhesion between layer 10 and thin film 12 in the uncovered areas, but not in the masked areas 12b. As a result, the masked areas 12b are preferential areas with a high bond strength of intimate contact so that layer 10 and thin film 12 will not separate. When peeling or bending takes place, the anodic or oxide thin film 12 is detached from the underlying metal layer 10 in the uncovered areas 12a, but remains attached to the underlying metal layer 10 in the masked areas 12b because of the tenacious adhesion of the anoidic thin film 12 to the metal layer 10 in these areas. Masked areas 12b continue to generate the original intense color, while uncovered areas 12a undergo the color change. As a result, the latent indicia becomes visible to indicate tampering.
Similar labels are commercially available from Minnesota Mining and Manufacturing Company, in St. Paul, Minn. as 3M.TM. Optical Thin Film Authenticating Devices. FIG. 4 is an illustration of such a label. Label 20 includes a release liner 24, a layer of adhesive 26, a flexible substrate 16, layer 10, thin film 12 and translucent layer 14.
Although the performance of available interference color generating devices has been impressive, it is desirable to further improve the device by allowing authentication without removing the device from the article .